1. Field of the Invention
The present invention generally relates to a computer system, and more particularly, to a power management circuit and a power management method of the computer system.
2. Description of Related Art
The universal serial bus (USB) standard is an expansion bus standard broadly adopted by personal computer (PC) systems. Many existing USB devices support USB charging (for example, a PC allows a USB device, such as a cell phone, to charge its battery). Either a desktop PC or a notebook computer can supply power to its USB ports in the standby state (S3) and the soft-off state (S5) specified in the advanced configuration and power interface (ACPI) standardin order to support USB charging.
As to a USB device to be charged, the charging process should be as fast as possible, and the current supplied by the USB port should be as large as possible. However, the power supply unit of a general desktop PC supplies merely a current of at most 2 A in aforementioned standby state or soft-off state. If a USB device extracts a charging current greater than 2 A in the standby state or the soft-off state, the standby voltage of the power supply unit will be too low to allow the PC to boot up from the standby state or the soft-off state.
In a notebook computer supporting USB charging, there is an embedded controller (EC) which requires constant power supply in order to determine whether the power of a battery is adequate, so as to determine whether the computer system possesses the lowest power to boot up. However, when no device to be charged is plugged into the USB port, the EC requires constant power supply. As a result, the battery durability of the notebook computer is reduced.